1. Field of the Invention
The present invention relates to a liquid crystal display element, a method of manufacturing the element, and electronic paper having the element.
2. Description of the Related Art
Recently, various enterprises and universities are actively engaged in the development of electronic paper. The most promising field of application of electronic paper is electronic books, and other promising fields include the field of portable apparatus such as mobile terminal sub-displays and display sections of IC cards. One type of display elements used in electronic paper is liquid crystal display elements utilizing a cholesteric liquid crystal component which forms a cholesteric phase (such a component is called a cholesteric liquid crystal or chiral nematic liquid crystal and will hereinafter be referred to using the term “cholesteric liquid crystal”). A liquid crystal display element utilizing a cholesteric liquid crystal has excellent characteristics such as semi-permanent display retention characteristics (memory characteristics), vivid color display characteristics, high contrast characteristics, and high resolution characteristics.
FIG. 13 schematically depicts a sectional configuration of a liquid crystal display element 51 capable of color display utilizing a cholesteric liquid crystal. The liquid crystal display element 51 has a structure in which a blue (B) display portion 46b, a green (G) display portion 46g, and a red (R) display portion 46r are formed one over another in the order listed from the side of the element where a display surface is provided. The display surface is located on the side of the element where a top substrate 47b is provided, and external light (indicated by the arrow in a solid line) impinges on the display surface from above the substrate 47b. An eye of a viewer and the viewing direction of the viewer (represented by the arrow in a broken line) are schematically depicted above the substrate 47b. 
The B display portion 46b includes a blue (B) liquid crystal layer 43b sealed between a pair of substrates, i.e., the top substrate 47b and a bottom substrate 49b and a pulse voltage source 41b for applying a predetermined pulse voltage to the B liquid crystal layer 43b. The G display portion 46g includes a green (G) liquid crystal layer 43g sealed between a pair of substrates, i.e., a top substrate 47g and a bottom substrate 49g and a pulse voltage source 41g for applying a predetermined pulse voltage to the G liquid crystal layer 43g. The R display portion 46r includes a red (R) liquid crystal layer 43r sealed between a pair of substrates, i.e., a top substrate 47r and a bottom substrate 49r and a pulse voltage source 41r for applying a predetermined pulse voltage to the R liquid crystal layer 43r. A light absorbing layer 45 is disposed on a bottom surface of the bottom substrate 49r of the R display portion 46r. 
The cholesteric liquid crystal used in each of the B, G, and R liquid crystal layers 43b, 43g, and 43r is a liquid crystal mixture obtained by adding a relatively great amount of chiral additive (which is also called a chiral material) to a nematic liquid crystal until a chiral material content of several tens percent by weight is reached. When a nematic liquid crystal includes a relatively great amount of chiral material, it is possible to form a cholesteric phase that is a strong helical twist of nematic liquid crystal molecules.
A cholesteric liquid crystal has bistability (memory characteristics), and the liquid crystal can be put in any of a planar state, a focal conic state, and an intermediate state that is a mixture of the planar state and the focal conic state by adjusting the intensity of an electric field applied to the same. Once the liquid crystal enters the planar state, the focal conic state, or the mixed or intermediate state, the liquid crystal thereafter remains in the state with stability even if the electric field is removed.
The planar state can be obtained by applying a predetermined high voltage between a top substrate 47 and a bottom substrate 49 to apply an intense electric field to a liquid crystal layer 43 between the substrates and thereafter nullifying the electric field abruptly. The focal conic state can be obtained by applying a predetermined voltage lower than the above-described high voltage between the top substrate 47 and the bottom substrate 49 to apply an electric field to the liquid crystal layer 43 and thereafter nullifying the electric field abruptly.
The intermediate state that is a mixture of the planer state and the focal conic state can be obtained by, for example, applying a voltage lower than the voltage to obtain the focal conic state between the top substrate 47 and the bottom substrate 49 to apply an electric field to the liquid crystal layer 43 and thereafter nullifying the electric field abruptly.
A principle behind a display operation of the liquid crystal display element 51 utilizing a cholesteric liquid crystal will now be described referring to an example of the B display portion 46b. FIG. 14A depicts alignment of liquid crystal molecules 33 observed when the B liquid crystal layer 43b of the B display portion 46b is in the planar state. Depicted as FIG. 14A, in the planar state, the liquid crystal molecules 33 are sequentially rotated from one another in the direction of the thickness of the substrates to form a helical structure, and the helical axes of the helical structure are substantially perpendicular to the substrate surfaces.
In the planar state, light rays having wavelengths in accordance with the helical pitch of the liquid crystal molecules 33 are selectively reflected by the liquid crystal layer. A wavelength λ which results in the maximum reflection is given by an equation λ=n·p where n and p represent the average refractive index and the helical pitch of the liquid crystal, respectively.
Therefore, in order to allow blue light to be selectively reflected by the B liquid crystal layer 43b of the B display portion 46b in the planar state, for example, the average refractive index n and the helical pitch p are determined such that an equation “λ=480 nm” becomes true. The average refractive index n can be adjusted by selecting the liquid crystal material and the chiral material appropriately, and the helical pitch p can be adjusted by adjusting the chiral material content.
FIG. 14B depicts alignment of the liquid crystal molecules 33 observed when the B liquid crystal layer 43b of the B display portion 46b is in the focal conic state. Depicted as FIG. 14B, in the focal conic state, the liquid crystal molecules 33 are sequentially rotated from one another in an in-plane direction of the substrate surfaces to form a helical structure, and helical axes of the helical structure are substantially in parallel with the substrate surfaces. In the focal conic state, the B liquid crystal layer 43b loses the selectivity of wavelengths to be reflected, and most of light rays incident on the layer are transmitted. Since the transmitted light rays are absorbed by the light absorbing layer 45 disposed on the bottom surface of the bottom substrate 49r of the R display portion 46r, a dark state (black) can be displayed.
In the intermediate state that is a mixture of the planar and focal conic states, the ratio between reflected light and transmitted light is adjusted according to the ratio between the planar state and the focal conic state to vary the intensity of reflected light. Thus, intermediate gray levels can be displayed according to intensities of reflected light thus obtained.
As thus described, the amount of light reflected by the cholesteric liquid crystal can be controlled by using an alignment of the helically twisted liquid crystal molecules 33. The liquid crystal display element 51 capable of color display is fabricated by enclosing cholesteric liquid crystals selectively reflecting green light and red light in the planar state in the G liquid crystal layer 43g and the R liquid crystal layer 43r, respectively, in the same manner as done in the above-described B liquid crystal layer 43b. The liquid crystal display element 51 has memory characteristics, and the element can perform color display without consuming electric power except when rewriting a screen.
However, a liquid crystal display element utilizing a cholesteric liquid crystal has a problem in that a state of display memorized therein can change when an external force is applied, e.g. when a display surface of the element is pressed or bent. In the case of a TN (Twisted Nematic) or STN (Super Twisted Nematic) liquid crystal display element, the liquid crystal is normally in an electrically driven state. Therefore, even when there is a change in the state of display, the previous state of display can be immediately restored. However, in the case of a liquid crystal display element utilizing a cholesteric liquid crystal, the cholesteric liquid crystal is not driven except when a screen rewrite is performed. Therefore, once the state of display of a liquid crystal display element utilizing a cholesteric liquid crystal changes, the previous state of display cannot be restored until the liquid crystal is driven again. The most significant advantage of a liquid crystal display element utilizing a cholesteric liquid crystal is the property of memorizing a state of display. Therefore, the above-mentioned problem constitutes a significant bottleneck to be overcome to put a liquid crystal display element utilizing a cholesteric liquid crystal in practical use.
Patent Document 1: JP-A-10-307288
Patent Document 2: JP-UM-58-13515
Patent Document 3: JP-A-8-76131
Patent Document 4: JP-A-2000-147527
Patent Document 5: JP-A-2002-82340
Patent Document 6: JP-A-2004-219948